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Oct. 7, 1947. K. L. BURGENER CYCLIC CONTROL MEANS FOR LINE RELAYS 14 Sheets-Sheet 1 Original Filed Feb. 9, 1945 IN VEN TOR.

o5. .228 m2: Ill 22225 .IIII Il 555mm o2 .228 @2.285 m2: E22 fom KARL L. BURGENER oPomJmm @25950 owomzzoo BY 40mm,

Atfornevs Oct. 7, 1947. K. L. BURGENER 2,428,550

CYCLIC CONTROL MEANS FOR LINE RELAYS Original Filed Feb. 9, 1945 14 Sheets-Sheet 2 FIG. FIG. FIG. FIG. FIG. FIG. FIG. FIG FIG. FIG. FIG. FIG, FIG. I5

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/Zl IVIAIN EXCHANGEZO OIITCOINC C 2W 203 LI E I-swITNCH SELECTOR r C2I TOLL BOARD 206 TOLL LINE TO j f |10GALL ANSO" l OISTANT OFFICE 2I6 Egg Il Jaca TRUNK 7 2O CIR. 220

INCOMING 480\ SELECTOR ZIO CONNECTOR SELECTOR 249A CONNECTOR I 2l L LINE swITCII 2I4 O62a I OSI1 LINE F) INVENTOR. SWITCH 2l5 KARL I... BURGENER Attorneys Oct. 7, 1947. K L BURGENER 2,428,550

CYCLIC CONTROL MEANS FOR LINE RELAYS Original Filed Feb. 9, 1945 14 Sheets-Sheet 3 FIG. 3

MAIN EXCHANGE 2O 3 l/-o-p. BIM/H M347 R516 TRUNK CIRCUIT BOO REVERSING kga 'Il-@V324 M25-l com R'aspm 35o 86H v INVFJVTOR. KARL I BURGENER Attorneys Oct. 7, 1947.

K. BURGENER 2,428,550

CYCLIC CONTROL MEANS FOR LINE RELAYS Original Filed Feb. 9, 1945 FIG. 4

14 Sheets-Sheet 4 Switch X9/vlog? T1423 Select INVENTOR. KARL L. BURGENER Attorneys Oct. 7, 1947. K, B-URGENE 2,428,550

CYCLIC CONTROL MEANS FOR LINE RELAYS Original Filed Feb. 9, 1945 14 Sheets-Sheet 5 C505 fw 540i-@f2 TRUNK CIR 500 Marginal BRANCH EXCHANGE 50 80 535 547 5557EA R274 TiMER C 576\ o INVENTOR. FIG. 5 KARL L. BURGENER Attorneys OUCH UH UUJ Oct. 7, 1947. K, 1 BURGENER 2,428,550

CYCLIC CONTROL MEANS FOR LINE RELAYS Original Filed Feb. 9, 1945 14 Sheets-Sheet 6 fc3 WV-o-F/azl TRUNK CARCUIT SOO Harm; "v /y o4 I C621' 622 Glai I T\522 554./

Ll-Ans. Rea-fo \Polar W x @5A-4* \c675 669\ T/GV lels e46 3 L w J66| Y song c6751 .l eww-@41' v eeh ' INVENTOR. FIG. 6 KARL BURGENER BY Awwfgymcm Attorneys Oct. 7, 1947. K. l.. BURGENER 2,428,550

CYCLIC CONTROL MEANS FOR LINE RELAYS Original Filed Feb. 9, 1945 14 Sheets-Sheet 7 :3fm 700 F IG 7 70| 703 705; g1r2706 TRUNK CIR. 500 m 702E 704 l 74| Une 7 5 743 4 ,/-J 7v6\\ R770 p- A 7l5 LZ2/L 716% 7 067A Hold 773' Reverse swatchI 733'* l. L[limo 753 R710 R720 764 #YV c675\ 734\ C782\ A :if-745 'II-Oj-N7l7 INVENTOR. KARL l.. BURGENER BY 9m-M.M%L

A'r'forneys Oct. 7, 1947.

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CYCLIC CONTROL Original Filed Feb. 9, 1945 omen me@ BURGENER 2,428,550

MEANS FOR LINE RELAYS 14 Sheets-Sheet 8 BRANCH EXCHANGE 5C) russa* ma@ (C787 Finder 8H Selector BIO I ANS.

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L802 Busy ifi 480 @Ans Leos Conn Finder 1 t Line Gir.

Party L'ine Connector or oc Ton Straight Line Incoming Trunk Circuit 855 INVENTOR KARL L. BURGENER Attorneys Oct. 7, 1947.

K. L. BURGENER 5831 Cn H905 CYCLIC CONTROL MEANS FOR LINE RELAYS Original Filed Feb. 9, 1945 14 Sheets-Sheet 9 FG 9 @GRD CIRCUIT 900 N-# OC. 7, 1947. K L BURGENER 2,428,550

CYCLIC CONTROL MEANS FOR LINE RELAYS Original Filed Feb. 9, 1945 14 Sheets-Sheet l0 RIOSO Polar;` JR |020 Rlolo l c975\ Recall* Flasher C1044 INVUVTOR. FIG. IO KARL L. BURGENER BY MMM%J5 Attorneys Oct; 7, 1947.

K. L.'BURGENER CYCLIC CONTROL MEANS FOR LINE RELAYS Original Filed Feb. 9, 1945 LEEVE uzoVL '28 Sw' 14 Sheets-Sheet l1 upervisory HZ4' NVENTOR.

KARL L. BURGENER FIG. u @Y Attorneys Seamn Kom Oct. 7, 1947. K BURGENER 2,428,550

CYCLIC CONTROL MEANS FOR LINE RELAYS Original Filed Feb. 9, 1945 14 Sheets-Sheet 12 4 CORD CIRCUIT SOO Attorneys Oct. 7, 1947. K, L, BURGENER 2,428,550

CYCLIC CONTROL MEANS FOR LINE RELAYS Original Filed Feb. 9, 1945 14 Sheets-Sheet 13 Distributor |335 anual E. M305 f |31 IBSI |365 |362 w 394f` h' INVENToR. FIG. l5 I KARL LBURGENER Attorneys oearcn KOU Oct. 7, 1947. K. l.. ABURGENER cYcLIc CONTROL MEANS Fon LINE RELAYs riginal Filed Feb. 9, 1945 14 Sheets-Sheet 14 Patented Oct. 7, 1947 benign au CYCLIC CONTROL MEANS FOR LINE RELAYS Karl L. Burgener, Villa Park, Ill., assignor to Automatic Electric Laboratories, Inc., Chicago, Ill., a corporation of Delaware Original application February 9, 1945, Serial No. 576,913. Divided and this application October 20, 1945, Serial No. 623,545

Claims.

The present invention relates to telephone systems in general and, more particularly, to lmprovements in interexchange telephone networks including both automatic and combined automatic and manual exchanges. This application is a division of the copending application of Karl Burgener, Serial No. 576,913, iiled February 9,

It is an object of the present invention to provide in a telephone system of the character indicated, an improved dialing arrangement for use in controlling the auomatic switching arparatus comprising an impulse receiving relay which is disconnected from the dialing circuit each time the circuit thereof is interrupted durirg pulsing, thereby preventing extraneous current surges in the dialing circuit from affecting the impulse receiving relay, and includes a circuit for preenergizing the impulse receiving relay during this period so that the relay will recperate rapidly after it is reconnected to the dialing circuit and tha interrupted dialing circuit is reclosed.

In accordance with another object of the invention, provisions are made whereby the automatic subscribers in the main automatic exchange may establish toll connections by directly controlling the automatic switching apparatus to extend the connections to the toll switchboard, and whereby the automatic subscribers in the combined automatic and manual exchange who attempt to setup toll connections to the toll switchboard by directly contro-lling the main exchange automatic switching apparatus are rerouted to the operator switchboard at the combined automatic and manual exchange.

The invention, both as to its organization and method of operation, together with other objects and advantages thereof. will best be understood by reference to the following specication taken in connection with the accompanying drawings, in which Figure 1 illustrates the general arrangement of a telephone system embodying the pres i ent invention; Figs. 2 to 14, inclusive, taken to-A gether, illustrate the details of a system having incorporated therein the features of the invention, as briefly outlined above; and Fig. 15 illustrates the mode of combining Figs. 2 to 14, inclusive, of the drawings to form a unified system. More specifically, Figs. 2, 3 and 4 show a schematic layout of the equipment located in the main exchange 20, including the detailed circuits of a two-way trunk circuit 300; Figs. 5, 6 and 7 show the detailed circuits of a two-way trunk circuit 500 located in the branch exchange 50; Figs. 8 and 13 show the schematic layout of additional 2 equipment located at the branch exchange 50 and includes the detailed circuits of certain line equipment; Figs. 9 to 12, inclusive, show the details of a cord circuit 900 at the manual switchboard |000; and Fig. 14 shows the details of an operator telephone circuit associated with the cord circuit 90D.

Referring now more particularly to Fig. 1 of the drawings, the telephone system there illustrated comprises a main exchange 20 and a private branch exchange 50, referred to hereinafter as the branch exchange 50. The main exchange 20 terminates a number of automatic subscriber lines which may be trunk connected to subscriber lines served by the branch exchange 50 and includes a toll switchboard 206 terminating a number of toll trunks from distant exchanges, which may be interconnected with the subscriber lines served by the main exchange automatic equipment or which may be interconnected with subscriber lines served by the branch exchange equipment.

The branch exchange 50 terminates a number of automatic subscriber lines which may be trunk connected through appropriate automatic switching equipment with automatic subscriber lines terminating in the main exchange 20 and includes a manual switchboard terminating manual subscriber lines which may be interconnected with the automatic subscriber lines terminating in either the branch exchange 50 or the main exchange 20. .The branch exchange also includes automatic switching apparatus whereby certain of the automatic subscriber lines of the system may establish connections directly with certain manual subscriber lines, and whereby connections with other manual subscriber lines may be established with the aid of the operator at the manual switchboard |000.

The in exchange 20 and the branch ex- `change-51|. are a so onnectd by a plurality' of two-way trunk lines,such as 50 I, whereby/ccnne Maen.- immaginava-...0.9.111- p l eted either with or without massfligguhe optors'tthe svvtfgnbigfs provided in the re'- sp c exc igwefs'las willbE'explainedlmore fully here a er. The two exchanges are also interconnected by a plurality of one-way trunk lines, such as and 86|, the former being provided to enable the operators at the branch exchange switchboard |000 to extend connections to automatic subscriber lines terminated in the automatic switching apparatus located in the main exchange 20, and the latter being provided to intercept connections which automatic subscribers in the branch exchange 50 attempt to extend to the toll switchboard in the main exchange 20 by wrongfully by-passing the manual switchboard in the branch exchange 50.

CALL FROM THE SUBsTA'rIoN A IN THE MAIN Ex- CHANGE 20 To THE MANUAL SUBsTArIoN H IN THE BRANCH EXCHANGE 50 In order for the subscriber at substation A in the main exchange to establish a connection with the subscriber at substation H in the branch exchange, the subscriber at substation A must rst call the branch exchange operator at the manual switchboard |000. After ascertaining the number of the called subscriber the operator thereat will then complete the connection by plugging into the jack terminating the desired called manual line. For example, when the subscriber at substation A removes his receiver, the line switch 20| operates in a well known manner and selects an idle selector switch. Assuming that the selector switch 202 is the one selected by the line switch 20|, the selector 202 responds and causes a dial tone signal to be transmitted to the subscriber at substation A in order to indicate that the selector switch is in condition to respond to the impulses dialed by the calling subscriber.

Since the desired called substation is located in the branch exchange 50, the digit to be dialed by the calling subscriber at substation A corresponds to the level in the selector 202 which has access to the group of trunk lines terminating in the manual switchboard |000. When the calling device at the substation A is operated in accordance With this digit the selector 202 will elevate its wipers to the selected level, after which, the wipers of the selector automatically rotate to select an idle trunk line in the selected group of trunks. In the present instance it Will be assumed that digit 3 is assigned to this group of trunks. Consequently, when the subscriber at substation A dials this digit the selector 202 responds and raises its Wipers to the third level of its associated set of bank contacts and then automatically rotates its wipers over the selected level to engage the contacts terminating an idle trunk line in the selected group of trunks. Accordingly, it will be assumed that the wipers of the selector 202 have selected the trunk line 204 comprising conductors C220, 022|, C222 extending to the trunk circuit 300 of Figs. 3 and 4.

It may be Well to note at this time that the trunk circuit 300 of Figs. 3 and 4 may be selected by a calling main exchange subscriber over either one of two paths. If the trunk circuit 300 is seized over the trunk line 204 it operates, in a manner to be described hereinafter, to extend a connection to the branch exchange at the manual switchboard |000. However, if the trunk circuit 300 is seized over the trunk line 203 it operates in a manner, to be described hereinafter, to extend a connection to the automatic switching apparatus in the branch exchange 50 so that the calling subscriber in the main exchange may extend a connection to a desired automatic subscriber in the branch exchange without the intervention of a manual operator.

Since the trunk circuit 300 in the present instance has been seized by the selector 202 over the trunk line 204, the line relay R430 and the select relay R410 are now operated over a series circuit including the conductors C220 and 022|,

and the selector 202 applies ground potential to conductor C222 of the trunk line 204 and to the conductor C2| 9 of the trunk line 293 mill' tiples thereof in the banks of other selectors in order to mark the trunk circuit 300 busy to all selectors having access thereto. The circuit for operating the line relay R430 and the select relay R410 may be traced from ground, by way of the upper winding of the line relay R430, the winding 342 of the repeating coil 340, the contacts 322 and 3|4, the winding of the select relay R410, the conductor C22|, through the selector 202, the line switch 20|, one conductor of the subscriber line 2|, the closed loop circuit at the substation A, and returning over the other conductor of the subscriber line 2| through the line switch 20|, the selector 202, the conductor C220, the contacts 3 |3 and 32 I, the winding 34| of the repeating coil 340, and the lower winding of the line relay R430, to battery. Upon operating, the selector relay R410, at its contacts 41|, opens a point in the circuit of the outgoing trunk 400 extending to the incoming selector 2|0 in order to prevent operation thereof. At its contacts 412, the relay R410 extends ground potential by way of the contacts 45| to the conductor C502, in order to unbalance the trunk line 50| and cause the trunk circuit 500 to extend the call to the branch exchange switchboard |000, in a manner to be described hereinafter. The line relay R430, upon operating over the above described circuit, at its contacts 432 completes an operating circuit for the hold relay R440 and at its contacts 43| it prepares a circuit for controlling the trunk circuit 500.

When the hold relay R440 operates over the above mentioned circuit, at its contacts 44| it completes a circuit for operating the switching relay R450, and it completes an energizing circuit for the right-hand polarizing winding of the supervisory relay R4|0. The supervisory relay R4|0 is a shunt field electro-polarized relay and does not operate its contacts 4|| when only its right-hand polarizing winding is energized. However, when the current iiow in the left-hand operating winding of the relay is in opposition to the current now in the right-hand polarizing winding of the relay, the contacts 4|| are operated, as will be described hereinafter. At its contacts 443, the relay R440 applies ground potential to the control conductors C2 9 and C222. Ground potential is applied to the control conductor C222 from the trunk circuit 300 in order to replace the busy ground potential applied thereto for a short period of time by the selector 202. This ground potential now maintains the selector 202 and the line switch 20| in their operated positions and marks the trunk line 204 as busy to other selectors having access thereto. Ground potential is also applied to the control conductor C2|0 in order to mark the other trunk line 203 as busy to the selectors having access thereto. At its contacts 444, the relay R440 prepares a circuit for operating the control relay R320, and it also prepares a circuit, including the resistor 341 and the contacts 33|, for preenergizing the lower winding of the line relay R430. The latter circuit is effective to increase the speed at which the line relay R430 responds when the trunk circuit 300 operates to repeat impulses received therein.

When the switching relay R450 operates over the above mentioned circuit, at its contacts 45| and 453 it disconnects the conductors C502 and C503 from the incoming selector 2|0 and, at its contacts 452 and 454, it connects the conductors C502 and C503 to the right-hand windings of the repeating coil 340. At its contacts 456, the relay B450 places a shunt circuit around the winding Sltil licor;

of the select relay R410, thereby causing the latter relay to restore and also to reduce the resistance in the incoming circuit in order to insure more positive operation of the line relay R430l At its contacts 455, the relay R450 also prepares a point in the circuit for subsequently operating the relay R460.

Since the connection being described is one which is to be extended by way of the branch exchange operator at the manual switchboard |000, further impulses are not transmitted by the calling subscriber and, consequently, the line relay R430 remains in its operated position.

It will be recalled that when the select relay R410 was initially operated, ground potential was applied at its contacts 412 to the conductor C502 and, as a result thereof, a circuit was completed for operating the differential relay R650 and the line relay R640 in the trunk circuit 500 in the branch exchange 50. This circuit may be traced from ground by way of the contacts 412 and 45|, the conductor C502, the contacts 6| I, the winding 10| of the repeating coil 100, the contacts 1|2 and 66|, the lower winding of the differential relay R650, and the lower winding of the line relay R640, to battery. Since no circuit is completed for the opposing upper winding of the differential relay R650 at this time, the relay operates to close its contacts 65| and completes a circuit for operating the signal relay R630. The line relay R640, upon operating over the above traced circuit, at its contacts 64| prepares a circuit for operating the reversing relay R660. At its contacts 643, the relay R640 completes a holding circuit for the signal relay R630 which may be traced from ground, the contacts 643 and 633, and the winding of the relay R630, to battery.

It will be recalled that after the ground potential is applied to the conductor C502 at the contacts 412, the switching relay R450 energized and at its contacts 452 and 454 it connected the conductors C502 and C503 to the right-hand windings of the repeating coil 340 in the trunk circuit. When the above described operation takes place, a circuit may be traced from ground by Way of the upper winding of the line relay R640, the upper winding of the differential relay R650, the contacts 662 and 1|4, the winding |02 of the repeating coil 100, the contacts 6|2, the conductor C503, the contacts 454 and 43|, the winding 344 of the repeating coil 340, the left-hand winding of the supervisory relay R4|0, the winding 343 of the repeating coil 340, the contacts 452, the conductor C502, the contacts 6| the winding 10| of the repeating coil 100, the contacts 1|2 and 66|, the lower winding of the diierential relay R650, and the lower winding of the line relay R640, to battery. It will be noted that the above traced circuit includes the two windings of the relays R640 and R650 in series with the left-hand winding of the supervisory relay R4|0; the latter relay, however, is not operated over this circuit since the current flow through its left-hand winding is in opposition to the current ilow through its right-hand polarizing winding. The line relay R640, however, remains operated over this circuit but the differential relay R650 is now restored to normal, because the current flow through its upper winding is in opposition to the current flow through its lower winding. Upon restoring, the relay R650, at its contacts 65|, opens the initial energizing circuit for the signal relay R630, but this relay remains in its operated position over a holding circuit including its contacts 633.

Referring again to the operation of the line relay R640, at its contacts 644 it also applies ground potential to the conductor C611 by way of the contacts 532, thereby causing the operation of the hold relay R130. As a further result of the operation of the relay R640, at its contacts 642, it opens a point in the incomplete circuit of the connect relay R620; at its contacts 645 it opens a point in the incomplete circuit of the reversing relay R140; and at its contacts 646 it opens a point in the incomplete circuit of the switching relay R1 l 0.

As has been mentioned above, the signal relay R630 is operated as a result of the operation of the differential relay R650 and it locks itself in its operated position over a circuit including its contacts 633 and the contacts 643. As a further result of the operation of the relay R630, at its contacts 632 it disconnects the cutoff relay R6|0 from the Cl conductor extending by way of the cable 580 to the incoming selector 830. At its contacts 634, the relay R630 completes a circuit for energizing the relay R520 which may be traced from ground, by way of the contacts 634, the resistance 60|, the contacts 6I4, and the winding of the relay R520, to battery. At its contacts 635, the relay R630 completes a circuit from ground by way of the contacts 663 and 635, the conductor C616 and the answer lamp L803, to battery. The answer lamp L803 is illuminated as a result of the above traded circuit to indicate to the operator at the switchboard |000 that a call has been received and should be answered. At its contacts 63| the relay R630 also completes a circuit for transmitting ringback tone to the calling subscriber in order to indicate that the operator at the branch exchange 50 is being signaled. This circuit may be traced from the ring-back tone conductor C62l, the condenser 622', the contacts 63|, 664' and 6|2, the conductor C503, the contacts 454 and 43|, and the winding 344 of the repeating coil 340, whereat the ring-back tone is repeated in a well known manner, to the calling subscriber.

When the hold relay R operates as a result of the ground potential applied t0 the conductor C611, at its contacts 13| it prepares a circuit to be described hereinafter; at its contacts 132 it places ground potential on the control conductor C181 in order to busy the trunk circuit 500 in the banks of all outgoing selectors to prevent the trunk circuit from being seized for an outgoing call; at its contacts 133 it completes an energizing circuit for the left-hand polarizing winding of the shunt eld relay R610; and, at its contacts 134, it places ground potential on the conductor C182, thereby to illuminate the busy lamp L802 and indicate to the operators at the switchboard |000 that the trunk circuit 500 is busy,

When the relay R520 operated over the above traced circuit under control of the signal relay R630. at its contacts 52| it disconnected the relays R550 and R560 from the TS conductor of the cable 580 extending to the incoming selector 830. The relay R520, at its contacts 522, completes an obvious circuit for energizing the control relay R540. Upon operating, the control re1ay`R540, at its contacts 542 and 543, disconnects the and -lconductors of the cable 580 extending to the incoming selector 830 from the trunk conductors C502 and C503; and, at its contacts 54| and 544, it prepares a circuit including the and conductors of the cable 580, which circuit, however, is ineffective in the present call. As a further result of the operation of relay R540, a

multiple circuit is completed, at its contacts 545, for the relay R520, which circuit is independent of the contacts 6I4 of the cutoff relay R6|0. At its contacts 546, the relay R540 completes a circuit for the relay R520 which includes the resistance 58| and the ground potential applied therethrough by way of the contacts 133; and, at its contacts 541, it disconnects an incomplete circuit including the interrupter 536 to prevent the answer lamp L803 from being flashed at the present time.

From the foregoing description it will be understood that the calling subscriber at substation A has now extended the connection to the manual switchboard |000, that the manual operator thereat has been signaled by the illumination of the answer lamp L803l and that the calling sub.. scriber has been notified of the fact that the operator is being signaled by the transmission of the ring-back tone signal.

It may be well to mention at this time that when the operator has taken her position at her section of the switchboard |000 and is ready to handle calls, she must first insert the plug of the operator headset |400 at that position into the jack terminating the operator telephone circuit. Thus to answer and extend calls received at the switchboard |000 the headset plug |403 is inserted into the jack |404. In response to this operation the transmitter I 40| is included in a series circuit with the battery feed impedance coil |405 and in multiple with the left-hand winding |43| of the induction coil |430. Furthermore, the receiver |402 is bridged across the lower right-hand winding |434 of the induction coil |430 in series with the contacts |443 and |445 and the condenser |435. It should also be noted that a pair of oppositely poled copper oxide half-Wave rectiiiers |428 is included in a circuit shunting the receiver |402. These rectiers are connected in parallel in order to pass current in both directions and `therefore do not actually rectify. However, at the speech level of ordinary conversation the rectiers as a unit have a high resistance and do not introduce any appreciable loss to the speech currents traversing the circuit, but on the other hand they have a relatively low resistance to a large voltage change such as would produce a loud click in the receiver |402 and thereby practically short-circuit the receiver |402 to weaken the clicks to a point where they are no longer obiectionable to the operator.

Accordingly, when the operator is signaled by the illumination of lamp L803 indicating that a call is to be answered, she inserts the answer plug of one of her cord circuits into the jack associated with the lamp L803. It will be assumed that the operator has selected the cord circuit 900, of Figs. 9 to l2, inclusive, and that she has inserted the plug P900 into the jack J800.

When the plug P900 is inserted into the jack J 800 the series connected relays R950 and R910 in the cord circuit 900 and the reversing relay R660 in the trunk circuit 500 are connected over a Series circuit which may be traced from ground, by way of the windings of the marginal switching relay R950 and the sleeve relay R910, the contacts H28, the sleeve elements of the plug P900 and the jack .1800, the contacts 80|, the sleeve conductor C615, the contacts 64| arid 623, and the upper winding of the reversing relay R660, to battery. The sleeve relay R910 in the branch exchange 50 and the reversing relay R660 in the main exchange 20 operate over the above mentioned circuit but, due to the resistance included in this circuit, the switching relay R950 does not operate at this time.

In the main exchange 20 the reversing relay R660, upon operating at its contacts 661, completes a holding circuit including its lower winding, from ground at the contacts 643; and, at its contacts 66| to 664, inclusive, it reverses the connection of battery and ground potentials applie by way of the windings of the line relay R640 t the trunk line 50|. The reversal of the direction of current flow over the trunk line 50| causes the current flow in the left-hand Winding of the supervisory relay R4I0 to be in such a direction that the relay R4|0 is now operated to close its contacts 4| I. Thus at its contacts 4l I, the relay R4|0 completes an obvious circuit for operating the reversing relay R3I0. The reversing relay R3I0 upon operating, at its contacts 3II to 3I4, inclusive, reverses the direction of current ow over the conductors C220 and C22I of the trunk line 204 and the calling line 2|, which reversal may cause the operation of the calling subscriber meter, if desired. The relay R3|0, at its contacts 3 I 5, also applies a multiple holding ground potential to conductors C2I9 and C222 and to the control conductor of cable 480.

As a further result of the operation ofthe relay R660 in the trunk circuit 500, at its contacts 665 it completes a holding circuit, including the grounded sleeve conductor C615, for its upper operating winding, which circuit is independent of contacts 623 and 64|. At its contacts 668 and 662', the relay R660 completes separate multiple circuits for holding operated the relays R130 and R520, respectively. At its contacts 664', the relay R660 interrupts the circuit for transmitting ringback tone to the calling subscriber and, at its contacts 663', it interrupts the circuit for illuminating the answer lamp L803.

It will be recalled that the sleeve relay R910 of the cord circuit 900 operated over a previously traced circuit when the plug P900 was inserted into the jack .1800 to answer the call received over the trunk line 180. Upon operating over this circuit, the relay R910, at its contacts 91|, completes a circuit for bridging the left-hand operating winding of the shunt ileld relay R|020 across the tip conductor C184 and the ring conductor C183, by way of the tip and ring elements of the plug P900 and jack J800 to cause the operation of the line relay R110 in the trunk circuit 500, as will be described hereinafter. At its contacts 912, the relay R910 also completes an obvious circuit for the right-hand polarizing winding of the shunt field relay R|020, and, at its contacts 913, it prepares a point in the circuit of the recall relay RI030. At its contacts 914 and 915, the relay R910 prepares points in the control circuit of the answer lamp L9l9.

When the left-hand winding of the shunt field relay R|020 was bridged across the tip and ring conductors of the trunk line 180, a loop circuit was completed for operating the line relay R110 of the trunk circuit 500. This circuit may be traced from ground, through the upper winding of the line relay R110, the contacts 16| and 14|, the lower right-hand winding 104 of the repeating coil 100, the tip conductor C184 of the trunk 180, the tip elements of the jack J600 and plug P900, the contacts 902 of the key KI, the contacts 94|, 915 and 91|, the left-hand winding of the shunt eld relay R|020, the contacts 904 of the key K90 I, the ring elements of the plug P900 and jack J800, the ring conductor C183 of the trunk 180, the upper right-hand winding 103 of the repeating coll 100, the contacts 142 and 162, and the lower winding of the line relay R110, to battery. The relay R110, upon operating, at its contacts 11| prepares a point in the circuit for the right-hand winding of the shunt field answer relay R610; at its contacts 112 it interrupts a point in the incomplete circuit of relay R160; and, at its contacts 113, it applies ground potential to conductor C611 in order to maintain the hold relay R130 operated.

The shunt field relay R|020 in the cord circuit 900 also operates over the above traced circuit including the windings of the line relay R110. At its contacts |02|, relay R|020 closes a point in the operating circuit for the ringing relay R|240 and, at its contacts |022, it interrupts a point in the incomplete circuit for the answer lamp L9l9.

The operator at the switchboard |000 having answered the call by inserting the plug P900 into the jack J800, may now operate the talk key K|| in order to associate her operator telephone circuit (Fig. 14) with the selected cord circuit 900. When the talk key Kl |00 is actuated a circuit is completed for operating the talk relay RI |20. Upon operating, the talk relay RI |20, at its various contacts, associates the operator telephone circuit of Fig. 14 with the selected cord circuit 900. It will be noted, however, that the various conductors C| |0| to CI |2, inclusive, which are individual to the operator telephone circuit, are also common to all of the cord circuits available to the operator in order to permit the association of the operator telephone circuit with any one of the cord circuits by merely operating the talk key individual to a selected cord circuit.

Accordingly, when the relay RI |20 operates, at its contacts ||24 and ||25 it connects the tip conductor C184 and the ring conductor C183 of the trunk 180 to conductors C||04 and C||05, respectively, of the operator telephone circuit, whereupon a conversational circuit is completed including the contacts |44| and |448, the righthand windings |433 and |434 of the induction coil |430, and the receiver |402 of the operator headset |400. Accordingly, the voice currents from the calling subscriber line complete a circuit through the upper right-hand winding |433 and the lower right-hand winding |434 of the induction coil |430. The receiver |402 which is bridged across the lower right-hand winding |434, carries a greater part of the voice currents because of the low impedance path it oiers in comparison with the lower right-hand winding |434. When the operator speaks into the transmitter |40| the resistance of the transmitter varies, causing the current supplied from battery through the winding of the impedance coil |405 to vary at different voice frequencies. This fluctuation in current also passes through the left-hand winding |43| of the induction coil |430 but is kept out of the battery circuit by the impedance |405 to prevent cross talk. The current uctuation passing through the left-hand winding |43| induces a similar voltage in the windings |433 and 434 of the induction coil |430, and since the winding |434 has the same number of turns as the winding |433 the induced voltage across the winding |434 will be equal to that induced across the winding |433. Since the resistance of the winding |434 is approximately equal to the resistance of the winding |433 plus the loop resistance of the calling line, the voltage induced into the windings |433 and |434 produces current flow through the receiver of the calling station and the receiver |402 of the operator headset. It should also be noted that there are two paths through the receiver |402 that voice currents may follow. One is from the midpoint of the windings |433 and |434 of the induction coil |430, through the windings |434, the contacts |443, the receiver |402, the contacts |445, the condenser |435, and back to the midpoint between the windings |433 and |434. The second path may be traced from the midpoint of the windings |433 and |434, the condenser |435, the contacts |445, the receiver |402, the contacts |443 and |448, the conductor C||05, over the calling subscriber loop and returning by way of the conductor C| |04, the contacts |44|, the winding |443, and back to the midpoint between the windings |433 and |434. Since these two circuits have equal voltages induced in them and have approximately equal resistances, the resulting currents will be approximately equal. Because the direction of the current flow in these circuits is always opposite through the receiver |402, and being approximately equal in magnitude, the resulting current through the receiver will be quite small, thereby minimizing the voice currents through the receiver |402 when the voice currents originate from the transmitter |40| associated therewith.

At this point it may 'be well to note that transmission battery is supplied to the calling su-bscriber line 2| through the windings of the line relay R430 in the trunk circuit 300. Transmission battery for the trunk line 50| extending between the right-hand windings of the repeating coil 340 of the trunk circuit 300 and the lefthand windings of the repeating coil of the trunk circuit 500, is supplied through the windings of the line relay R640 in the trunk circuit 500. Transmission battery for the trunk line 180 interconnecting the right-hand windings of the repeating coil 100 of the trunk circuit 500 and the operator telephone circuit is supplied through windings of the line relay R of the trunk circuit 500. Accordingly, the operator at the switchboard |000 may now converse with the subscriber at the calling substation A in order to obtain the telephone number of the desired called manual subscriber at substation H.

Referring again to the operation of the talk relay RI |20, in addition to connecting the talking conductors of the calling trunk to the operator telephone circuit at its contacts ||24 and ||25, it also prepares, at its contacts ||22 and ||23, a circuit for connecting the tip and ring conductors of the call `plug P|210 to the operator telephone circuit by way of the conductors C| |02 and CI |03. At its contacts H28, the relay Rl |20 disconnects the sleeve conductor C915 from the circuit, including the series connected switching relay R950 and the sleeve relay R910, and in place thereof, at its contacts |21 and H26, sulbstitutes a circuit including the conductors C||08 and CI |09 and the release local key K|408 and the switch-through key K|4|5 in the operator telephone circuit. Accordingly, the circuit for the switching relay R950 and the sleeve relay R910 now includes the sleeve conductor C915, the contacts |21, the conductor C| |09, the contacts |4|0 of the key K|408, the contacts |4|1 of key K|4|5, the conductor C| 08, the contacts |26, the windings of relays R910 and R950, to ground. The control to be exercised by the keys K|408 and K|4|5, which are now included in series with the sleeve conductor C915 and the above mentioned relays, will Ibe described in detail hereinafter. At. its contacts ||20', ||2| and H22', the relay RI |20 inserts the contacts of the toll ring key K|420 into the circuit including the sleeve conductor of the call plug P|210 and the windings of the relays RI |40 and RI |50, for the purpose to Vbe described hereinafter. Furthermore, the relay Rll20, at its contacts H23', opens a point in the circuit of the dial-through key K|025 and, at its contacts H24', prepares a point in the circuit of the dial relay R|250.

Extending the call from the operatm position at switchboard 1000 to the called subscriber at substation H Before extending the connection to the called subscriber line the operator at the switchboard |000 may restore the talk key Kl to disconnect her operator telephone circuit from the cord circuit 900. This operation, however, is optional since the operator telephone circuit may be retained connected to .the cord circuit 900 during lthe setting up of a call, and may be disconnected therefrom after the connection is established. However, it will be assumed in the present case that the talk key Kl |00 is restored to normal -by the operator before the connection is extended.

A further operationto be performed Iby the operator before the connection is to be extended to the called line of substation I-I is to test the called line to determine whether or not it is busy in another connection by placing the tip of the plug PI210 against the sleeve element of the jack J |30| terminating the called subscriber line. If the called line is busy, a ground potential will be applied to the sleeve element of the jack J ill through the rectifier unit ||35 of the cord circuit which is used in interconnecting the busy subscriber line of the substation H with another subscriber line. Consequently, when the tip of the plug P|20 is placed into engagement with the sleeve element of jack J |30I, the ground potential thereon will be extended over a circuit which may be traced through the tip element of the plug P|210, the contacts |252, |24| and ||8|, the winding of supervisory relay RI |10, the contacts and ||4|, the conductor CIIUI, the resistance |493, and the winding |432 of the induction coil |430, to battery. This circuit 'produces a click in the receiver |402 of the operator headset |400 to indicate to the operator that the line of substation H is busy. When this condition exists the operatormust reoperate the talk key KI |00 to reconnect her telephone circuit to the cord circuit 900 and the calling subscriber line in order to inform the calling subscriber that the called subscriber at substation H is busy.

It will now be assumed that when the above described 'busy test is made lby the operator the line is idle and, therefore, battery potential applied to the sleeve conductor of the jack J through the winding of the cutoi relay R|320 does not produce a click in the receiver of the operator telephone circuit.

In order for the operator at the switchboard |000 to extend a call to the manual substation H, the operator inserts the plug P|210 into the jack J|30| terminating the line |321 individual to the manual substation H. When the plug P|210 is inserted in the jack J |30|, a circuit is completed for operating in series the sleeve relay RI |40, the switching relay RI |50. and the cutoff relay nlm, the nner remy being individua to the called line |321, over a circuit which may be traced from ground. by way of the winding of 2 relay Rl |50, the winding of relay RI |40, the con@ tacts I |22', the sleeve elements o! the plug PIZIE and the jack J |30|, and the winding of relay R|320, to battery. The resistance in the above described circuit is such that the three relays R||40, R||50 and R|320 all operate to perform various control operations to be described hereinafter.

The relay RI |50, upon operating over the above traced circuit, at its contacts ||5| and ||54 interrupts points in the tip and ring conductors of the cord circuit; at its contacts H50' it opens a point in the incomplete circuit of the relay RI 230; at its contacts H55 and H51 it prepares separate operating and holding circuits for the control relay RIIBD; and, at its contacts H58, it completes a circuit for energizing only the lower polarizing winding of the relay R|220.` This energizing circuit merely prepares the relay for quick operation when a circuit including its upper operating winding is completed. Further, the relay R||50 also connects, at its contacts H50, a circuit including the winding of the relay R|2 Il, the upper winding of the relay Rl220, and the contacts |263 in bridge of the talking conductors of the cord circuit 900 (indicated by heavy lines) extending to the tip and ring elements of the plug P|210. One talking conductor'of the cord circuit 900 extends to the tip element of the plug P1210 through the contacts ||8|, |24| and |252, and the other talking conductor of the cord circuit extends to the ring element of the plug P|210 through the contacts H03, |243 and |254. At its contacts H59, the relay R||50 also prepares a further point in the operating circuit of the ring relay R|240.

The relay RI |40, upon operating over the previously traced circuit, interrupts, at its contacts H43 and H45, a point in the incomplete circuit of the relay R|060. Although the relay R|000 is bridged across the talking conductors of the cord circuit 900 through the contacts H52 and H53, this relay cannot be operated at the present time because it is disconnected from the answering end of the cord circuit at the contacts and 954, and it is also disconnected from the calling end of the cord circuit at the contacts |l5| and H54. At its contacts II4I, the relay RI |40 also disconnects the busy test circuit extending to the operator telephone circuit; at its contacts H40 it prepares a point in the incomplete circuit of the answering recall relay RI040; and, at its contacts H41, it completes a circuit for illuminating the call lamp LI 21|. The circuit for illuminating the call lamp may be traced from ground, through the contacts 9|8, 966, H64 and H41, and the call lamp LIZ'H, to battery. As a further result ot the operation of the relay R||40 another point in the incomplete operating circuit of the ring relay R|240 is prepared at its contacts H40, and, at its contacts H49, a further point is closed in the incomplete operating circuit of the relay Ri'230.

As a result of the above described operations of the relays Rl |40 and RI |50, the cord circuit 900 is now in condition to be controlled by the operator at the switchboard |000 to signal the wanted subscriber at substation H. However, before this operation is described it may be well to mention that as a result of the seizure of the called line of substation H by the insertion of the plug P|210 into the jack J 30|, the cutoff relay R|320 of the wanted subscriber line circuit |300 is operated. At its contacts |32| and |322, the relay R|320 disconnects battery and ground po- 

